Porous aluminosilicate compounds such as zeolites have found utility in a variety of industrial processes such as gas separation, catalysis, and petroleum processing. Structurally, aluminosilicates include alternating SiO4/AlO4 tetrahedra linked together through bridging oxygen atoms, which create a 3-dimensional network with cages and/or channels of uniform size. These cage and channel structural features have been identified as useful for imparting the particular chemical and catalytic properties to the particular aluminosilicate.
Sodalite, shown in FIG. 1A, is a simple variant of the zeolite class of aluminosilicates. Sodalite's aluminosilicate network forms an assembly of small β-cage units in the shape of truncated octahedra, containing four and six-membered rings of alternating SiO4/AlO4 tetrahedra. The resulting β-cage stoichiometry, [(AlSiO4)3]3, requires charge balancing cationic species within the cage. These charge balancing M+ cations are Na+ in natural sodalite, which occupy a single crystallographic site, coordinated to the framework oxygen atoms in the six-membered rings, as illustrated in FIG. 1B. In hydrosodalite, Na6(AlSiO4)6.8H2O, each cage contains three Na+ cations and four water molecules (the formula reflects the crystallographic unit cell which consists of 2 β-cage units). In dehydrated sodalite, Na6(AlSiO4)6, all water molecules are removed from the cages.
Sodalites, like more complex aluminosilicate zeolites, are able to exchange their associated charge balancing cations with other cations, a process that is traditionally carried out in aqueous solution. These cation-exchanged aluminosilicate reactions generally are undertaken to alter or tailor the properties of the particular aluminosilicate to achieve a desired activity or structural feature. However, when using aqueous ion exchange methods, sodalite and various other zeolites may tend to exchange hydronium (H3O+) ions from hydrolyzed water or from acidic hydrated metal complexes that can form with transition metal and rare earth cations in aqueous solution. This process, in turn, can lead to the breakdown of the aluminosilicate framework and/or can produce undesirable metal hydroxide precipitates on the surface of the aluminosilicate.
Therefore, new methods for effecting cation-exchange reactions and processes that are applicable to sodalite and various other zeolites, and other types of aluminosilicates, are needed that may be less susceptible to side reactions and produce fewer adverse byproducts. Further, generally more robust ion exchange methods are needed to produce the desired ion exchanged materials. Desirably, such methods would be useful with a variety of zeolites or other aluminosilicates. Therefore, there remains a need for new methods and new synthetic approaches for preparing cation-exchanged aluminosilicates.